Design of Inductors With Significant AC Flux
The design methodology shown in Fig. 1 is introduced to select a minimum core volume for an inductor or coupled inductors experiencing appreciable core loss. The geometric constant (Kgac) has been found to be a power function of the core volume for commercial toroidal, ER, and PQ cores, permitting the total loss to be expressed as a direct function of the core volume. The inductor is designed to meet specific loss or thermal constraints. An iterative design procedure is used, in which 2D or 3D proximity effects are neglected at the beginning, and then subsequently incorporated via finite element simulation. The methodology is demonstrated in Fig. 2 for coupled inductors in a 5-MHz converter operating in critical conduction mode. The core loss is veri-fied using rectangular excitation, and the winding loss is inferred from the noise robustness of thermal measurements and low thermal resistance. Negative couplings between conductors as well as a compact layout with 2.89-nH common-source inductance in the module eliminates self-turn-on from 420-A/μs di/dt.